1. Field of the Invention
The present invention relates to a technology for reducing and stabilizing a beam spot size on a scanning surface in an optical scanning device.
2. Description of the Related Art
Recently, improvement in image quality of images output by “image forming apparatuses, such as multifunction peripherals (MFPs)” through optical scanning is being more and more demanded. To meet such a demand, a beam spot size at the time of optical scanning needs to be reduced and stabilized. “The stabilization of the beam spot size” can be attained by expanding a depth allowance of a light beam (a defocusing distance in an optical axis direction, which is determined such that a beam spot size is maintained within an allowable range). Regarding the depth allowance, it is widely known that the following relationship is satisfied.d∝w2/λwhere d is depth allowance, w is beam spot size, and λ is operation wavelength for optical scanning. In other words, if the depth allowance increases, the beam spot size increases in proportion to the depth allowance. Therefore, it has been difficult to attain both reduction and stabilization of the beam spot size at the same time.
One approach to “expand a depth allowance” and maintain a small beam spot size at the same time may be to use a Bessel beam.
A “Bessel beam” is disclosed in, for example, Japanese Patent No. 3507244. Specifically, it is disclosed that the Bessel beam has a side lobe with an extremely high light intensity and a high-order side lobe with a relatively high light intensity. Due to such characteristics, if the Bessel beam is used for optical scanning, light use efficiency is lowered, which makes it difficult to increase a processing speed of an image forming operation to the recently-required level.
Environmental fluctuation also affects an image forming operation performed through optical scanning. If environmental conditions such as a temperature or humidity change in an optical scanning device, a wavelength of a laser light source varies or optical characteristic changes due to thermal deformation of a lens. As a result, a focal position of a scanning light beam on a focusing surface fluctuates. Conventional technologies for reducing a shift of a focal position on a focusing surface by using a diffraction lens are disclosed in, for example, Japanese Patent Application Laid-open No. 2005-258392 and Japanese Patent Application Laid-open No. 2006-235069.